This paper describes the influence of organic fertilizers on the content of vitamin C, carotenoids, antioxidants and heavy metals (Cd, Cu, Pb) in two onion cultivars, i.e., “Stuttgarter Riesen” and “Rote Laaer”. The experiment was done during 2016 and 2017. Treatments B-Stimul (contains Azospirillum, Azotobacter, Bacillus, Chlorella vulgaris and Herbaspirillum), Ekobooster 2 (contains organic matter and NPK stimulates) and Vermifit A (composition nutrients in immediately acceptable forms, plant hormones, enzymes, extract of compost of californian earthworm) were used. The results showed that the cultivar “Stuttgarter Riesen”at the treatment B-stimul in 2016 reached the highest content of vitamin C (35.4 mg kg^-1 and carotenoids (14.6 mg kg^-1), while in 2017 the highest content of vitamin C (44.4 mg kg^-1) and antioxidants were (4.1 mM TE.100 g^-1). The cultivar “Rote Laaer” reached the highest content of carotenoids in both years in the treatment Ekobooster 2, 7.5 mg kg^-1 (2016) and 22.8 mg kg^-1 (2017). In 2016, control showed the highest content of vitamin C (17.5 mg kg^-1) and antioxidants (18.5 mM TE. 100 g^-1). In both years and in both cultivars, the content of Cd and Pb was in line with European law, and the content of Cu was lower than those recommended by EU standards.